1. Field of the Invention
The present invention relates generally to telemetric systems and more particularly to devices for adjusting the electromagnetic field of a telemetric system.
2. Related Art
Wireless technology in devices such as pagers and hand-held instruments has long been exploited by the healthcare sector. However, skepticism of the risks associated with wireless power and communication systems has prevented widespread adoption, particularly in orthopaedic applications. Now, significant advances in microelectronics and performance have eroded many of these perceived risks to the point that wireless technology is a proven contender for high integrity medical systems. Today's medical devices face an increasingly demanding and competitive market. As performance targets within the sector continue to rise, new ways of increasing efficiency, productivity and usability are sought. Wireless technology allows for two-way communication or telemetry between implantable electronic devices and an external reader device and provides tangible and recognized benefits for medical products and is a key technology that few manufacturers are ignoring.
Telemetry systems typically comprise a single coil transmitter for transmitting electromagnetic signals and a receiver for receiving the electromagnetic signals from the transmitter. These coils are normally arranged in a parallel configuration. The telemetry data might be any physical measurement, such as implant load, implant micro-motion, alkalinity, temperature, pressure, etc., which is sensed by a remote telemetry unit.
Currently, Radio Frequency (RF) telemetry and inductive coupling systems are the most commonly used methods for transmitting power and electronic data between the implant and the companion reader. Radio telemetry system employs one of several different modulation methods and one of several different multiplexing techniques in order to transmit information produced by multiple sensors to a user's location remote from the sensors. Methods of modulating a carrier frequency to convey information include amplitude modulation, frequency modulation, phase modulation, and pulse modulation.
The short range of conventional telemetry devices is a potential limitation of telemetric implants for medical applications. In the medical monitoring field, continuously accessible telemetry over a longer range has been sought. Increased read range, however, has implications on the reader system in terms of power, size and cost. As best seen in FIG. 1, effective read range typically has a logarithmic relationship with power consumption. In the graph illustrated in FIG. 1, a read range of approximately 11 inches requires about 100 watts of power.
U.S. Pat. No. 5,630,835 discloses a telemetry receiver coil arrangement of the generic type mentioned above. The '835 Patent discloses an electronic apparatus for the non-invasive communication with a cardiac pacemaker. An antenna arrangement with two coils connected in opposite directions is provided in the electronic apparatus. This antenna arrangement with two coils connected in series in opposition of phase helps suppress the effects of so-called far-field interference signals on the received near-field signals from the implant.
U.S. Pat. No. 5,741,315 teaches, in a telemetric data transmission device, to dispose a receiver coil and a compensation coil of differing diameters on a cup-shaped support such that inhomogeneous signals—namely the signals emitted by the implant—are observed only in the receiver coil, whereas homogeneous signals—namely interference signals—are measured also in the compensation coil.
U.S. Pat. No. 6,405,088 relates to a telemetry coil arrangement for receiving data signals in particular from cardiologic implants, comprising a pair of coils disposed one after the other in the direction of reception.
There remains a need in the art for improved telemetry systems and particularly a need for devices to adjust the electromagnetic field of a telemetric system.